Mass spectrometer ion source with a two region ionization chamber to minimize energy spreading of the ions



Aug. 12, 1969 H. w.- w. WERNER 3,461,285- I MASS SPEG'I'ROMETER IONSOURCE WITH A TWO REGION IONIZATION CHAMBER 1'0 MINIMIZE ENERGYSPREADING OF THE IONS Filed June 28. 1967 2 Sheets-Sheet 1 v IIIIIIIIIIIIIII 'IIIIIIIIIIIIIII;

. t y I F'IGCI INVENTOR.

HELMUT w. w. wsnnsn BY AGENT Aug. 12, 1969 v H. w'. w. WERNER 3.461285--MASS SPECTROMETER ION SOURCE WITH A TWQ REGION *IONIZATION CHAMBER T0MINIMIZE ENERGY SPREADING OF THE IONS Fil ed June 28. 1967 2Sheets-Sheet 2 INVENTOR.

HELMUT w. w; wenusn BY AGEM United States Patent 3,461,285 MASSSPECTROMETER ION SOURCE WITH A TWO REGION IONIZATION CHAMBER TO MINIMIZEENERGY SPREADING OF THE IONS Helmut Wilhelm Werner Werner, Emmasingel,Netherlands, assignor to US. Philips Corporation, New York, N.Y., acorporation of Delaware Filed June 28, 1967, Ser. No. 649,544 Claimspriority, application Netherlands, July 2, 1966, 6609292 Int. Cl. B071159/44 US. Cl. 250-41.9 7 Claims ABSTRACT OF THE DISCLOSURE An ion sourcefor a mass spectrometer in which ions are formed by ionization of a gasto be tested with an electron beam collimated by a magnetic field in thedirec-;

tion of the beam, the ions formed being extracted from the ionizationspace through a small gap parallel to the direction of the electron beamby means of an electric field the direction of which extends at rightangles to the direction of the electron beam, a fiat grid-like electrodebeing provided consisting of parallel wires which cross the direction ofthe electron beam, the latter electrode dividing the ionization spaceinto two zones one of which contains the ionization gap, a potentialbeing applied to the grid-like electrode which in combination withpotentials applied to the other electrode produces a small voltagegradient in the first zone and a large voltage gradient in the otherzone.

The invention relates to an ion source, in particular an ion source fora mass spectrometer.

In a known ion source ions are formed by ionization of a gas to betested by means of an electron beam which is collimated by a magneticfield in the direction of the beam, the ions formed being extracted fromthe ionization space through a small gap parallel to the direction ofthe electron beam by means of an electric field the direction of whichextends at right angles to the direction of the electron beam. In such asource the electrons emitted by a filament are confined into a narrowbeam with a comparatively weak magnetic field having values up to a fewhundred Gauss. Under the influence of a weak electric field which is atright angles to the beam the ions which are formed are extracted fromthe ionization space through a gap into an ion lens. The extractingfield which is of the order of magnitude of a few volts per centimeteris supplied by the potential of the first electrode ot the ion lens, thepotential of the wall of the ionization space, and the potential of arepeller electrode inside the ionization space. The repeller electrodeis located so that the electron beam traverses between said electrodeand the emanating gap. When the ions have traversed the ion lens theypass through a slit in the analyzer space of the spectrometer. Theanalysis takes place by means of a sector-like magnetic field.

In such an ion source there will always be mass dis- 3,461,285 PatentedAug. 12, 1969 crimination because of the collimating magnetic field inthe ionization space. In fact, the ion paths in the crossed electric andmagnetic fields are cycloids in a plane at right angles to the directionof the magnetic field in which the factor ME occurs as a parameter whereM is the mass number of the eB /i0n, E is electric field strength of theextracting field, B the magnetic field in the ionization space, and ethe charge of the ion. The particles having the smallest mass (M) willfollow the most curved paths. Particles having a small mass (M) will notpenetrate to the analyzer because they do not pass the gaps in the wallof the ionization space, or reach too far beyond the axis of the ionlens. This latter effect may be prevented by choosing the potentials ofdeflection electrodes in the ion lens to match the relative mass (M)value. However, this matching is very time consuming and is associatedwith loss of other mass (M) values. The mass discrimination could bechecked by increasing the extracting field strength, i.e., by applying alarger voltage diiference to the extraction electrodes. However, thishas an adverse influence on the electron beam and also increases thevoltage gradient across the ionization area, i.e., the area where theelectron beam is, which results in an increased energy spreading of theions and consequently an undesired decrease of the resolving power ofthe spectrometen.

It is a principal object of the invention to provide an improvement ofion sources which are thus constructed.

It is a further object of the invention to reduce the massdiscrimination as a result of the collimating magnetic field in an ionsource for a mass spectrometer.

It is still further an object of the invention to increase theefficiency without increasing the energy spreading in the ionizationarea of an ion source for a mass spectrometer.

These and further objects of the invention will appear as thespecification progresses.

According to the invention, in an ion source for a mass spectrometer ofthe above-described type a flat grid-like electrode consisting ofparallel wires which cross the direction of the electron beam and thatof the extracting field at right angles is arranged at a distance of afew tenths of a millimeter from the ionization area. This electrodedivides the ionization space into two areas the first of which comprisesthe emanating gap. A potential is applied to that electrode which, incombination with the potentials of the extracting electrodes, produces asmall potential gradient which is a maximum of a few volts percentimeter in the first area, and produces a large potential gradientwhich is at least some tens of volts per centimeter in the second area.

These differences in gradient are produced by the potentials of theelectrodes having the following orders of magnitude.

The repeller electrode has a small positive potential with respect tothe grid which is not more than a few volts. The flat front wall of theionization space in which the emanating gap is located has a negativepotential with respect to the grid of the order of tens of volts.Consequently, a strong field which is formed as a result of the gridand, possibly, also by the potentials of electrodes of the ion lensbetween the grid and the front wall does not penetrate too much into thearea which is restricted by the grid and the housing of the ionizationspace which consists of a flat rear wall opposite to the emanating gapand a box-like part of the wall which has the rear wall as a base.

In one embodiment the repeller electrode is connected to the housing andthe grid is insulated from the surrounding parts.

In another embodiment the grid is connected to the housing and therepeller electrode is insulated from the surrounding parts.

In another embodiment both the repeller electrode and the grid areconnected to the housing. The ions are then extracted from theionization area by the penetrating field of the front wall which has anegative potential with respect to the housing of a few hundred volts.

The grid is supported by a holder. Preferably a construction is used inwhich the wall of the said holder is,

not at right angles to the plane in which the grid is located but widensin the direction of the emanating gap. As a result of this a certainextent of focussing of the ions in the emanating gap can be obtained.

In each of the above-described embodiments in the area between therepeller electrode and the grid the ions need cover only a very shortdistance, namely at a maximum the thickness of the ionization area plusthe distance from the ionization area to the grid. At the grid thedeviation as a result of the collimating magnetic field is still small.In the area between the repeller electrode and the grid such a smallpotential gradient prevails that the energy spreading of the ions is notlarger than in the conventional sources. In the area between the gridand the front wall a large potential gradient prevails, i.e., a strongfield strength so that ions with different masses describe heresubstantially identical paths substantially parallel to the axis of theion lens as a result of which this area does not contribute to massdiscrimination and the efliciency is increased.

In each of the above-described embodiments laborious adjustment of theion lens to efficiently pass particles the masses of which are locatedin a given range through the ion lens is not necessary. Potentialdifferences between defiection electrodes on either side of the axis ofthe ion lens which, in the absence of the grid, must be of the order of100 volts, need no longer be used when the grid is employed. Thesepotentials need be a maximum of a few volts now and apparently serveonly to remove field disturbances as a result of irregularities in theelectrodes.

' It has furthermore been found that in the above-described embodimentsthe ion yields for all masses vary linearly over a large range as afunction of pressure in the ionization chamber with one singleadjustment of the ion lens.

The invention will now be described in greater detail with reference tothe accompanying drawings which are drawn substantially entirely toscale.

FIGURE 1 is a cross-sectional view normal to the electron beam of a partof the ion source and the ion lens inside the vacuum envelope.

FIG. 2 shows part of FIG. 1 on an enlarged scale, equipotential linesalso being shown.

In FIG. 1 the cross-section of the electron beam is denoted by the area1 in which the ions are formed. In a direction along the axis of theelectron beam which has a current strength of approximately 100 ,ua.,the proportions of this area gradually increase from 2 mms. 0.5 mm. to 4mms. 2 mms. The cross-section shown in FIG- URE l is chosen to be suchthat the upper limit of these dimensions 4 mms. 2 mms. is shown.

The collimating magnetic field has a strength of 150 Gauss and isdenoted by B in the figure. The ionization space is limited by a flatfront wall 2 which comprises an emanating gap 3, having a length of 10mms. and a width of 1 mm., and a housing consisting of a flat rear wall4 and a box-like part 5. The ionization space comprises a repellerelectrode 6 and a grid-like electrode 7 which is located at a distanceof 8 mms. from the rear wall 4, at a distance of 2.5 mms. from the frontwall 2 and at a distance of 0.5 mm. from the area 1. The grid is securedto a molybdenum frame and consists of tungsten wires, 25,11. thickness,mutual distance approximately The wires are arranged at right angles tothe direction of the electron beam in order to prevent the structure ofthe grid from appearing in the ion spectrum.

The grid is supported by a holder 8 which widens in the direction of thegap 3.

The ion lens is constituted by the electrodes 9, 10, 11, 12, 13, 14 and15.

The electrode 15 includes a slot 16 affording access to the analyzerspace.

In FIG. 2 the numbers in brackets indicate the potentials with respectto the housing on the lines and electrodes at the applied voltages. Therepeller electrode 6 is at a potential of 0 volts with respect to thehousing of the ionization space. The grid 7 is at a potential of l.7volts with respect to the housing. The front wall 2 is at 30 volts withrespect to the housing. The electrode pair 9 and 10 is at 500 volts withrespect to the housing and the electrode 11 is at -2000 volts withrespect to the housing. The potential of the housing with respect toground is determined by the potential difference between the electrodes11 and 15 from FIG. 1, the latter of which is at ground potential.

In the area 1 the potential gradient at these voltages is only 3.5 voltsper centimeter, but in the area between the grid 7 and the front wall 2the potential gradient at these voltages is volt per centimeter.

Voltage differences exceeding 5 volts on oppositely located deflectionelectrodes (9 and 10; 13 and 14) need not exist in the ion lens, noteven to cause particles having a mass of 1 mass unit to reach theanalyzer.

While the invention has been described with reference to specificembodiments and applications thereof, other modifications will bereadily apparent to those skilled in the art without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In an ion source for a mass spectrometer, the combination of ahousing closed at one end by a wall having an aperture therein for thepassage of ions, means within said housing for producing an electronbeam in a given direction for ionizing a gas in said housing, means toproduce a magnetic field in said given direction, means to produce anelectric field in said housing in a direction perpendicular to saidgiven direction for extracting ions through said aperture, a grid-likeelectrode within said housing having parallel wire-like elementsintersecting the direction of the electron beam, said grid-likeelectrode being positioned between said wall having the apert-uretherein and said electron beam and being spaced further from said wallthan from said electron beam, said grid-like electrode being spaced adistance from said electron beam of the order of tenths of a millimeter,said grid-like electrode dividing the interior of said housing into twozones one of which contains the electron beam, means to produce apotential gradient in a first zone containing the electron beam notexceeding the order of volts per centimeter, and means producing apotential gradient in the other zone of the order of tens of volts percentimeter.

2. An ion source as claimed in claim 1 in which a repeller electrode ispositioned in the first zone on the opposite side of the electron beamfrom the grid-like electrode, a potential being applied between saidelectrodes for producing the potential gradient in said first zone.

3. An ion source as claimed in claim 2 in which the 5 repeller electrodeis connected to the housing and the grid-like electrode is insulatedtherefrom.

4. An ion source as claimed in claim 2 in which the grod-like electrodeis connected to the housing and the repeller electrode is insulatedtherefrom.

5. An ion source as claimed in claim 2 in which the repeller electrodeand the grid-like electrode are connected to the housing.

6. An ion source as claimed in claim 2 in which the grid-like electrodeis supported by a conductive holder 1 which widens in the direction ofthe emanating gap.

7. Anion source as claimed in claim 1 including an ion lens in saidsecond zone in which potential differences exist between deflectionelectrodes on either side of the axis of the ion lens which are not morethan a few volts.

References Cited UNITED STATES PATENTS 5/1960 Benson et al 250-419 OTHERREFERENCES 0 vol. 38, No. 5, May 1967, pp. 621-624.

RALPH G. NILSON, Primary Examiner C. CHURCH, Assistant Examiner

